29 Input/output library [input.output]

29.10 File-based streams [file.streams]

29.10.3 Class template basic_­ifstream [ifstream]

29.10.3.1 General [ifstream.general]

namespace std { template<class charT, class traits = char_traits<charT>> class basic_ifstream : public basic_istream<charT, traits> { public: using char_type = charT; using int_type = typename traits::int_type; using pos_type = typename traits::pos_type; using off_type = typename traits::off_type; using traits_type = traits; // [ifstream.cons], constructors basic_ifstream(); explicit basic_ifstream(const char* s, ios_base::openmode mode = ios_base::in); explicit basic_ifstream(const filesystem::path::value_type* s, ios_base::openmode mode = ios_base::in);// wide systems only; see [fstream.syn] explicit basic_ifstream(const string& s, ios_base::openmode mode = ios_base::in); template<class T> explicit basic_ifstream(const T& s, ios_base::openmode mode = ios_base::in); basic_ifstream(const basic_ifstream&) = delete; basic_ifstream(basic_ifstream&& rhs); // [ifstream.assign], assign and swap basic_ifstream& operator=(const basic_ifstream&) = delete; basic_ifstream& operator=(basic_ifstream&& rhs); void swap(basic_ifstream& rhs); // [ifstream.members], members basic_filebuf<charT, traits>* rdbuf() const; bool is_open() const; void open(const char* s, ios_base::openmode mode = ios_base::in); void open(const filesystem::path::value_type* s, ios_base::openmode mode = ios_base::in); // wide systems only; see [fstream.syn] void open(const string& s, ios_base::openmode mode = ios_base::in); void open(const filesystem::path& s, ios_base::openmode mode = ios_base::in); void close(); private: basic_filebuf<charT, traits> sb; // exposition only }; }
The class basic_­ifstream<charT, traits> supports reading from named files.
It uses a basic_­filebuf<​charT, traits> object to control the associated sequence.
For the sake of exposition, the maintained data is presented here as:

29.10.3.2 Constructors [ifstream.cons]

basic_ifstream();
Effects: Initializes the base class with basic_­istream<charT, traits>(addressof(sb)) ([istream.cons]) and sb with basic_­filebuf<charT, traits>() ([filebuf.cons]).
explicit basic_ifstream(const char* s, ios_base::openmode mode = ios_base::in); explicit basic_ifstream(const filesystem::path::value_type* s, ios_base::openmode mode = ios_base::in); // wide systems only; see [fstream.syn]
Effects: Initializes the base class with basic_­istream<charT, traits>(addressof(sb)) ([istream.cons]) and sb with basic_­filebuf<charT, traits>() ([filebuf.cons]), then calls rdbuf()->open(s, mode | ios_­base​::​in).
If that function returns a null pointer, calls setstate(failbit).
explicit basic_ifstream(const string& s, ios_base::openmode mode = ios_base::in);
Effects: Equivalent to: basic_­ifstream(s.c_­str(), mode).
template<class T> explicit basic_ifstream(const T& s, ios_base::openmode mode = ios_base::in);
Constraints: is_­same_­v<T, filesystem​::​path> is true.
Effects: Equivalent to: basic_­ifstream(s.c_­str(), mode).
basic_ifstream(basic_ifstream&& rhs);
Effects: Move constructs the base class, and the contained basic_­filebuf.
Then calls basic_­istream<charT, traits>​::​set_­rdbuf(​addressof(sb)) to install the contained basic_­filebuf.

29.10.3.3 Assignment and swap [ifstream.assign]

void swap(basic_ifstream& rhs);
Effects: Exchanges the state of *this and rhs by calling basic_­istream<charT, traits>​::​swap(rhs) and sb.swap(rhs.sb).
template<class charT, class traits> void swap(basic_ifstream<charT, traits>& x, basic_ifstream<charT, traits>& y);
Effects: Equivalent to: x.swap(y).

29.10.3.4 Member functions [ifstream.members]

basic_filebuf<charT, traits>* rdbuf() const;
Returns: const_­cast<basic_­filebuf<charT, traits>*>(addressof(sb)).
bool is_open() const;
Returns: rdbuf()->is_­open().
void open(const char* s, ios_base::openmode mode = ios_base::in); void open(const filesystem::path::value_type* s, ios_base::openmode mode = ios_base::in); // wide systems only; see [fstream.syn]
Effects: Calls rdbuf()->open(s, mode | ios_­base​::​in).
If that function does not return a null pointer calls clear(), otherwise calls setstate(failbit) (which may throw ios_­base​::​failure) ([iostate.flags]).
void open(const string& s, ios_base::openmode mode = ios_base::in); void open(const filesystem::path& s, ios_base::openmode mode = ios_base::in);
Effects: Calls open(s.c_­str(), mode).
void close();
Effects: Calls rdbuf()->close() and, if that function returns a null pointer, calls setstate(failbit) (which may throw ios_­base​::​failure) ([iostate.flags]).